Method and system for island detection and anti-islanding protection in distributed power generation systems

ABSTRACT

An effective, yet relatively simple and inexpensive, method for detection of islanding in distributed power generation systems. Statistical analysis of the local line frequency, as measured at the distributed generator, is performed to detect when an island has been formed. The statistical characteristics of the local frequency are controlled by the grid when the distributed generator is not islanding. When an island is formed, however, frequency control switches to circuitry associated with the distributed generator. Because the statistical characteristics of the frequency control performed by the distributed generator are markedly different from those of the grid, the islanding condition can be detected and corrected.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 61/565,292, entitled METHOD AND SYSTEM FOR ISLANDDETECTION AND ANTI-ISLANDING PROTECTION IN DISTRIBUTED POWER GENERATIONSYSTEMS, and filed Nov. 30, 2012, said application being hereby fullyincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to distributed power generation systems,and more specifically, to detection and correction of unintentionalisland formation in distributed power generation systems.

BACKGROUND OF THE INVENTION

Distributed power generation, in which relatively small electricalgeneration sources (sometimes known as distributed energy resources orDERs) that may be primarily intended to serve a specific load areinterconnected with a larger-scale electrical power grid to enablesharing of excess power that they generate, is coming into increasinguse as a way of facilitating the use of renewable energy resources andimproving the overall reliability and efficiency of the power grid. Apersistent problem in distributed power generation systems, however, isthe hazard to personnel and equipment that occurs when a segment of thegrid containing a DER becomes unintentionally disconnected from theremainder of the grid containing the primary central generation source.When a segment is disconnected, the DER may continue to power thedisconnected segment, forming an “island.” This can endanger personnelwho may be working to restore connection of the segment with the gridand may wrongly assume the islanded segment is not energized. Further,voltage or frequency deviations in the islanded segment may damageelectrical equipment connected to the segment. For these reasons,industry standards such as IEEE 1547 have been developed that requiredistributed generators to detect unintentional island formation so thatappropriate corrective action can be taken in a timely fashion.

Prior attempts have been made at developing methods for detecting islandformation, and have included active, passive, and communications-basedmethods. None of these prior methods, however, have proven entirelysatisfactory. For example, communications-based methods many beineffective if communications are interrupted before an islanding eventoccurs. Some of the prior methods rely on complex signal processingtechniques, which to be effective, often require the use of expensive,high-powered computational equipment. Other active prior islandingdetection methods require the distributed generation system to inject aperturbation with a specific detectable signature. Such methods,however, require specialized equipment to generate, inject, and detectthe signature.

What is still needed in the industry is an effective, yet relativelysimple and inexpensive method for detection of islanding in distributedpower generation systems.

SUMMARY OF THE INVENTION

Embodiments of the present invention address the need in the industryfor an effective, yet relatively simple and inexpensive, method fordetection of islanding in distributed power generation systems.According to embodiments of the invention, statistical analysis of thelocal line frequency, as measured at the distributed generator, isperformed to detect when an island has been formed. The statisticalcharacteristics of the local frequency are controlled by the grid whenthe distributed generator is not islanding. When an island is formed,however, frequency control switches to circuitry associated with thedistributed generator. Because the statistical characteristics of thefrequency control performed by the distributed generator are markedlydifferent from those of the grid, the islanding condition can bedetected.

The new passive method of islanding detection according to embodimentsof the invention is tentatively named RoCoF-H, which stands for “Rate ofChange of Frequency-Histogram.” This method involves measuring andrecording the histogram of rate of change of frequency (RoCoF, alsodenoted df/dt), over a selected time period—in other words, the recenthistory of the rate of change of frequency—and then analyzing thehistogram to extract an index value that allows a determination ofwhether the system is islanded. This value is then used in ananti-islanding decision algorithm to either command disconnection of theDER if an islanding condition is indicated, or to allow the DER to“ride-through” without disconnection if an islanding condition is notindicated.

BRIEF DESCRIPTION OF THE FIGURES

The embodiments of the present invention may be more completelyunderstood in consideration of the following detailed description ofvarious embodiments in connection with the accompanying drawings, inwhich:

FIG. 1 is a diagrammatic depiction of an exemplary distribution feedersystem;

FIG. 2 is an exemplary histogram depiction of the expected distributionof the absolute value of df/dt in a grid-tied distributed generationsystem;

FIG. 3 is an exemplary histogram depiction of the expected distributionof the absolute value of df/dt in an islanded case;

FIG. 4 is a depiction of a histogram for grouping measured df/dt valuesinto bins according to an embodiment of the invention;

FIG. 5 is a histogram depiction of measured frequency change values in agrid-tied distributed generation system;

FIG. 6 is a histogram depiction of measured frequency change values inan islanded distributed generation system;

FIG. 7 is a histogram depiction of measured frequency change values in agrid-tied distributed generation system just after a large localswitching event; and

FIG. 8 is a block diagram of an anti-islanding control system for a DERaccording to an embodiment of the invention.

While the present invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the presentinvention to the particular embodiments described. On the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the present invention.

DETAILED DESCRIPTION

In FIG. 1, there is depicted a diagram of an exemplary distributionfeeder 10. Feeder 10 generally includes feeder series impedances 12,load blocks 14, 16, 18, 20, 22, 24, and distributed generators 26, 28,which are in this case, photovoltaic (PV) systems. Voltage sourceUtility V at the left, along with its source impedance Source Z,represent the grid from the standpoint of feeder 10. When feeder 10 isconnected to the grid (i.e. feeder breaker 30, recloser 32, andsectionalizer 34 are all closed), the frequency on feeder 10 iscontrolled by the grid. The frequency of the grid is determined by therotational speed of the large main-line generators such as coal,nuclear, hydropower and natural gas-fired plants associated with UtilityV. This rotational speed is regulated by governors on these plants.Because of the relatively slow action speed of these governors and thefact that the generators typically have large rotational inertia, therate of change of frequency when feeder 10 is grid-tied will usually bevery slow. When the grid connection of feeder 10 is lost and feeder 10becomes an island, as may happen for example if feeder breaker 30 opens,the local frequency on feeder 10 is controlled by distributed generators26, 28. Inverter-based distributed generators, such as PV systems,generally have very fast-acting frequency controls and no rotationalinertia. Further, even if distributed generators 26, 28, are powered byengines or other rotating machines, such relatively small engines ormachines as are typically used in distributed generation systems havemuch faster governor control and much lower rotational inertia than thelarge main-line plants employed to power the grid. Thus, when feeder 10is islanded, the rate-of-change of the frequency (df/dt) can be expectedto shift to a higher range.

In FIG. 2, there is depicted an exemplary histogram of the absolutevalues of df/dt that would be expected for feeder 10 under grid-tiedconditions. As depicted, most of the values are clustered near zero.Another peak is depicted at much higher |df/dt| values. This higher peakis caused by the way in which frequency is measured, and occurs becauselarge load or another switching event on feeder 10 will cause transientsin the frequency, and in |df/dt|, that are large in value but very shortin duration. Thus, as depicted, when feeder 10 is under grid control,there will be primarily very fast (from switching) and slow (from thegrid) frequency changes. Hence, under grid control, the distribution of|df/dt| is generally bimodal as depicted in FIG. 2. FIG. 5 depicts ahistogram plot of measured frequency changes (Hz/s) over a given timewindow in one of the systems modeled in simulation as describedhereinbelow, with the system in a grid-tied condition. As can be seen,the expected bimodal distribution is present.

In FIG. 3, there is depicted an exemplary histogram of |df/dt| as wouldbe expected for feeder 10 under islanding conditions. With feeder 10under control of distributed generators 26, 28, the frequency on feeder10 is controlled by inverter phase locked loops or generator governorsassociated with generators 26, 28. The frequency changes resulting fromthese controls are much faster than the grid frequency changesassociated with large generating equipment, but slower than the loadswitching transients also encountered when feeder 10 is grid-tied. Asdepicted in the histogram of FIG. 3, the mode of the distribution of|df/dt| becomes generally singular, and moves to a location between thetwo modes depicted in FIG. 2 when feeder 10 is under grid control. FIG.5 depicts a histogram plot of measured frequency changes (Hz/s) over agiven time window in one of the systems modeled in simulation asdescribed hereinbelow, with the system in an islanded condition. As canbe seen, the expected generally singular distribution is present.

According to embodiments of the invention, the differing distributionsof |df/dt| depending on whether feeder 10 is grid-tied or is islandedcan be used to detect an islanding condition. As depicted conceptuallyin FIG. 4, a coarsely divided histogram is used, wherein the x-axis isdivided into three bins, defined by predetermined threshold valuesdenoted ToB1 and ToB2. Bin 1 is defined near the zero value for |df/dt|,and is intended to encompass the slow frequency change events associatedwith a grid-tied condition, Bin 3 is intended to encompass the fastswitching transients associated with a grid-tied condition, and Bin 2which falls between the two is intended to encompass the islandingassociated values of |df/dt|. Hence, the values of thresholds ToB1 andToB2 are set such that the histogram groups depicted in FIG. 2 fall intoeither Bin 1 or Bin 3, and the histogram group depicted in FIG. 3 fallsinto Bin 2. The top value of Bin 3 is always ∞.

It will be appreciated that over any time period T, there will bedeterminable numbers of events that fall into each of Bin 1, Bin 2, andBin 3. Using observed values for the respective numbers of these eventsover the time period T, a bimodality index BI can be computed asfollows:

${BI} = \frac{\sum_{Bin2}X_{i}}{{\sum_{{Bin}\; 1}X_{j}} + {\sum_{{Bin}\; 3}X_{k}}}$where Bin1, Bin2, and Bin3 are the histogram bins with boundaries chosento correspond to the low, middle and high groupings depicted in FIG. 4and X_(j), X_(i), and X_(k) are the elements of those bins respectively.In the grid tied condition, nearly all of the |df/dt| values should fallinto Bin1 and Bin3, and BI≈0. After the island forms, there is morefrequency “jitter” in the island because of the DER frequency controls,some, but not all, of the |df/dt| values move into Bin2, and B>0. Forsystem-wide frequency events, many values will move into Bin2, and B>>0.

It will be appreciated that the threshold values ToB1 and ToB2 should beselected to maximize the probability that the desired “middledistribution” depicted in FIG. 3 falls within Bin 2. The inventor hasdetermined baseline values, usable under most circumstances, for ToB1and ToB2 of 3 milliHertz per second (mHz/s) and 8 mHz/s, respectively.Of course, it will be appreciated that these values may need to beadjusted, depending on the circumstances in individual applications. Forexample, to apply the method of the present invention to a system in alocation where the grid may not be as “stiff” or “strong” as in thecontinental United States, such as on one of the Hawaiian islands, bothToB1 and ToB2 values would generally be increased. Also, it will berecognized that the values of ToB1 and ToB2 used will be dependent tosome extent on the devices used to measure frequency, because of thefiltering used in, and frequency responses of, these devices. Forexample, if a phasor measurement unit (PMU) is used, the method of theinvention can still be used—the inventor has successfully tested themethod using data from PMUs made by Schweitzer EngineeringLaboratories—but ToB1 and ToB2 will depend on the frequency estimationtechnique used in the PMU. Some frequency measurement devices will besimply unsuitable for this technique because their frequency response istoo slow to catch many of the events in Bin 2 or any event in Bin 3.Hence, it is important to ensure that the frequency measurementapparatus used at least has a fast enough response to capture eventsthat would fall into Bin 3, such as switching transients.

It will also be appreciated that it is important in an island detectionmethod to balance detection effectiveness with some degree of false-tripimmunity—accordingly, it is desirable to carefully select the decisioncriterion for determining whether a given result indicates the DERsshould “ride-through,” or an unintentional island case, in which theDERs should disconnect or enter a “micro-grid” mode. For example, acomplicating factor can be the effect of large local switching events,such as a heavily-loaded large motor switched to the feeder. FIG. 7depicts a histogram plot of measured frequency changes (Hz/s) just aftersuch an event in one of the systems modeled in simulation as describedhereinbelow, with the system in a grid-tied condition. As can be seen, asignificant number histogram components that would fall into Bin 2 arepresent.

One preferred approach to address this is a technique developed by theinventor and called the “zero-time” method: over a given time window,the number of samples is determined for which BI=0, and the durationrepresented by those samples is summed and compared to the total windowduration. For example, if a five-second window is chosen, and the totalduration for which BI=0 is less than 1 second, an island would beindicated. These times (the five and 1 second times) can be adjusted toimprove response speed, if the system's properties will permit thiswithout a loss of selectivity. The inventor has found that an accurateindication of island formation for most systems is obtained when thevalue of BI is in the range of 0<BI<10 over more than 75% of a givenwindow duration. Selectivity in some cases can be enhanced by usingextremely high values of BI to suggest a system-wide event instead of anisland and command a ride-through of the DERs. Another method is to usethe average of BI over a shorter window, but this method is inferior inselectivity. Standard deviation of BI over a window (or standarddeviation of df/dt directly) may also be used, but this may besignificantly inferior in selectivity.

Table 1 below is a table presenting the results of exemplary simulationsusing the RoCoF-H method according to the invention and the “zero-time”criterion. Representative simulation results are reported on twofeeders. The two feeders were: (1) the IEEE 34-bus distribution feeder,which is unusually long, mostly overhead, and high-impedance; and (2) areal-world feeder of medium stiffness (medium impedance) and serving asuburban region. Modeling was performed using EMTP-RV andMATLAB/Simulink, and in the case of the real-world feeders, usingdetailed observed feeder data supplied by the electric utility servingthe feeders.

On each of the two feeders, four cases were simulated. The first twocases were chosen to represent difficult cases for island detection.Case A is a multiple-inverter case, in which many three-phase inverterswere added to the feeder until a generation-load match could beachieved. As one example, the number of three-phase inverters added tothe IEEE 34 bus system was 18. These inverters were spread along thefeeder because the inductance between the inverters is believed toexacerbate the loss of anti-islanding effectiveness in themulti-inverter case. Phase-phase balancing was achieved by addingsingle-phase inverters to the more heavily loaded phases.

Case B was a case involving a mixture of types of DER. From ananti-islanding perspective, the most difficult combination of DERsarises when inverter-based DERs are combined with synchronousgenerators, so that is the case that was selected here. Some of theinverters in the multiple-inverter case were removed to make room for asingle 1 MVA synchronous generator.

The latter two cases C and D do not involve islands, but instead areindicative of false-trip immunity (i.e., cases in which ride through isdesired). Case C was a desired ride-through case simulating a loss ofmainline generation resulting in a system-wide frequency event. In thiscase, it is highly desirable that the island detection method be able todistinguish this case from an unintentional islanded case and stayonline to support the system. For this simulation, the frequencytrajectory used was one measured during a major Italian blackout of2003, scaled to 60 Hz. To implement this frequency trajectory, aprogrammable variable-frequency source was created in EMTP-RV andprogrammed to follow this trajectory based on a lookup table.

Case D is another ride-through case, this one involving a major localswitching event such as mentioned above with respect to FIG. 7. Again,the anti-islanding system must be able to distinguish such an event froman island to avoid excessive false tripping. To simulate this event, aheavily-loaded 200-hp three-phase induction motor was switched directlyacross the line at a distal point on the feeder.

In each of the simulations, ToB1 was set to 3 mHz/sec, and ToB2 to 8mHz/sec. The frequencies were measured by a phase-locked loop.

TABLE 1 Feeder Test IEEE 34 Bus Standard Case Feeder Real-World Feeder AIsland Detected Island Detected B Island Detected Island Detected CSuccessful Ride-Through Successful Ride-Through D SuccessfulRide-Through Successful Ride-Through

As can be seen, the RoCoF-H method and “zero-time” criterion achievedthe desired result in each of the simulated cases for both feeders.

In use, the RoCoF-H method is applied to a DER coupled to a grid-tiedfeeder by first determining the appropriate ToB1 and ToB2 values to beused with the system. Although the baseline values of 3 mHz/s and 8mHz/s as discussed above are expected to be appropriate for use withmost systems, it may be necessary to adjust these values to fit theindividual characteristics of the feeder and the frequency control andmonitoring equipment used with the DER. It may be necessary to conduct asystem study in simulation and observe the histograms. The inventor hasdiscovered that while it is usually possible to determine the grid-tiedhistogram simply through measurements, it is often not possible toobtain the islanded histogram experimentally.

As depicted in block diagram form in FIG. 8, a processor 50 programmedwith an algorithm for grouping real-time frequency data according to thehistogram model of FIG. 4, with the predetermined ToB1 and ToB2 values,is communicatively coupled to the frequency measuring and control deviceof the DER 52 through communications link 54. The algorithm alsoincludes appropriate decision criteria such as the “zero-time” criteriondescribed above. The processor receives real-time frequencymeasurements, groups them into Bin 1, Bin 2 and Bin 3 of the histogrammodel over a time window, computes a bimodality index for the timewindow, and applies the decision criteria to determine whether anislanding condition exists. If an islanding condition is determined,processor 50 can command DER 52 to shut down or disconnect, oralternatively in appropriately equipped systems, can command a switch 56to direct the DER generated power to a local load 58 so that the systemcan operate in a local “micro-grid” mode, serving only local load 58.

The foregoing descriptions present numerous specific details thatprovide a thorough understanding of various embodiments of theinvention. It will be apparent to one skilled in the art that variousembodiments, having been disclosed herein, may be practiced without someor all of these specific details. In other instances, components as areknown to those of ordinary skill in the art have not been described indetail herein in order to avoid unnecessarily obscuring the presentinvention. It is to be understood that even though numerouscharacteristics and advantages of various embodiments are set forth inthe foregoing description, together with details of the structure andfunction of various embodiments, this disclosure is illustrative only.Other embodiments may be constructed that nevertheless employ theprinciples and spirit of the present invention. Accordingly, thisapplication is intended to cover any adaptations or variations of theinvention.

For purposes of interpreting the claims for the present invention, it isexpressly intended that the provisions of Section 112, sixth paragraphof 35 U.S.C. are not to be invoked unless the specific terms “means for”or “step for” are recited in a claim.

What is claimed is:
 1. A method of detecting island formation in adistributed power system including at least one distributed generatorinterruptably coupled to a grid through a feeder, the method comprising:using a frequency measurement device to compile a plurality of values offrequency change in the power system, measured over a predetermined timewindow; using a processor to group the plurality of values of frequencychange into a plurality of bins, a first one of the bins representing anexpected behavior of power frequency change when the feeder isgrid-tied, a second one of the bins representing an expected behavior ofpower frequency change when the feeder is islanded, and a third one ofthe bins representing another expected behavior of power frequencychange when the feeder is grid-tied; computing a bimodality index forthe grouped values of frequency change; using the processor to apply apredetermined decision criterion to the grouped values of frequencychange to determine whether the feeder is islanded or grid-tied, whereinthe decision criterion comprises determining whether a value of thebimodality index is in the range of between 0 and 10 over more than 75%of the time window; and automatically decoupling the distributedgenerator from the feeder if the feeder is determined to be islanded. 2.The method of claim 1, further comprising determining a first limitvalue of frequency change defining a boundary between the first andsecond bins.
 3. The method of claim 1, further comprising determining afirst limit value of frequency change defining a boundary between thefirst and second bins, and determining a second limit value of frequencychange defining a boundary between the second and third bins.
 4. Ananti-islanding control apparatus for use in a distributed power systemincluding at least one distributed generator interruptably coupled to agrid through a feeder, the apparatus comprising a processorcommunicatively coupled with a control system of the at least onedistributed generator and programmed with an algorithm for: grouping aplurality of values of frequency change in the power system, measuredover a predetermined time window, into a plurality of bins, a first oneof the bins representing an expected behavior of power frequency changewhen the feeder is grid-tied, a second one of the bins representing anexpected behavior of power frequency change when the feeder is islanded,and a third one of the bins representing another expected behavior ofpower frequency change when the feeder is grid-tied; computing abimodality index for the grouped values of frequency change; applying apredetermined decision criterion to the grouped values of frequencychange to determine whether the feeder is islanded or grid-tied, whereinthe decision criterion comprises determining whether a value of thebimodality index is in the range of between 0 and 10 over more than 75%of the time window; and decoupling the distributed generator from thefeeder if the feeder is determined to be islanded.
 5. The apparatus ofclaim 4, wherein the algorithm further comprises determining a firstlimit value of frequency change defining a boundary between the firstand second bins.
 6. The apparatus of claim 4, wherein the algorithmfurther comprises determining a first limit value of frequency changedefining a boundary between the first and second bins, and determining asecond limit value of frequency change defining a boundary between thesecond and third bins.
 7. An article comprising a machine-readablemedium storing instructions operable to detect and correct islandformation in a distributed power system, the power system including atleast one distributed generator interruptably coupled to a grid througha feeder, the instructions for: grouping a plurality of values offrequency change in the power system, measured over a predetermined timewindow, into a plurality of bins, a first one of the bins representingan expected behavior of power frequency change when the feeder isgrid-tied, a second one of the bins representing an expected behavior ofpower frequency change when the feeder is islanded, and a third one ofthe bins representing another expected behavior of power frequencychange when the feeder is grid-tied; computing a bimodality index forthe grouped values of frequency change; applying a predetermineddecision criterion to the grouped values of frequency change todetermine whether the feeder is islanded or grid-tied, wherein thedecision criterion comprises determining whether a value of thebimodality index is in the range of between 0 and 10 over more than 75%of the time window; and decoupling the distributed generator from thefeeder if the feeder is determined to be islanded.
 8. The article ofclaim 7, wherein the instructions further comprise determining a firstlimit value of frequency change defining a boundary between the firstand second bins.
 9. The article of claim 7, wherein the instructionsfurther comprise determining a first limit value of frequency changedefining a boundary between the first and second bins, and determining asecond limit value of frequency change defining a boundary between thesecond and third bins.